1. Technical Field of the Invention
The present invention relates to a transceiver having a steerable antenna and, more particularly, relates to a transceiver and a method of operation thereof having circuitry for controlling a steerable antenna.
2. Description of the Related Art
Ordinary portable transceivers, such as portable telephones and pagers are equipped with an antenna having a constant gain pattern. Because the antenna has a constant gain pattern, the energy at various horizontal and vertical locations transmitted from or received by the antenna is fixed and not variable. To provide different antenna patterns, transceiver antennas have been proposed with an array of two or more antenna elements. By using the two or more antenna elements, the antenna pattern can be steered. U.S. Pat. No. 5,303,240 issued to Borras et al. uses a steerable antenna to selectively direct energy transmitted from a transceiver into a particular direction. Directing transmit energy from a transceiver in a particular direction reduces interference by reducing the system gain in other directions and enhances the desired signal strength.
Satellite communications systems under development, such as the Iridium.RTM. satellite system, propose using L-Band communications in a combination of frequency division multiple access and time division multiple access (FDMA/TDMA) multiplexing to make the most efficient use of limited spectrum. This TDMA structure has frames with a framing time slot and with a plurality of transmit and receive time slots of short duration. These frames are preferably built on a 90 millisecond frame with four transmit and four receive time slots, each of a short 8.28 millisecond duration, and a framing time slot of 23.76 milliseconds. In time division multiple access satellite communication systems, the number of time slots made available per cell can be limited by the system. In such a system, for example, each cell has only one time slot for use.
Power measurements taken for different antenna patterns during the single time slot can disrupt voice or data traffic. Borras et al. make power measurements during non-traffic time slots. In a satellite system using only one time slot per cell, non-traffic time slots are not available for making measurements. An efficient technique for measuring signal quality and steering an antenna is needed for communicating on a single time slot in a cell with a satellite. A technique for making alternate antenna pattern decisions without disrupting traffic in time slots is also needed.
Satellite communications systems, such as the Iridium.RTM. satellite system, propose a constellation of 66 non-geosynchronus orbiting satellites. Each satellite will have a plurality of 48 individual beams or cells projected onto the earth's surface. As these satellites orbit around the earth, both the satellites and satellite beams will move. A stationary point on the earth's surface will cross boundaries between beams once approximately every 50 seconds. A new satellite will serve the stationary point on earth once approximately every 9 minutes. In radiotelephone systems, handoff decisions are typically made by measuring power from different cells. When different antenna patterns must be evaluated and chosen at the same time as beams and satellites evaluated and chosen, non optimum antenna pattern and poor handoff decisions could be made. An efficient technique for adapting the antenna during the voice or data traffic in the current beam and handing off a transceiver between different beams of the same satellite and different beams of different satellites is also needed.